The present invention relates to a method and apparatus for producing ultra-high purity oxygen from the separation of air. More particularly, the present invention relates to such a process and apparatus in which the air is first separated into nitrogen and oxygen rich fractions and then is further refined to separate hydrocarbons, argon and nitrogen from the oxygen rich fraction to produce the ultra-high purity oxygen. Even more particularly the present invention relates to such a method and apparatus in which the hydrocarbons are first removed from the oxygen rich fraction by rectification and then argon and nitrogen are separated by stripping the oxygen rich fraction.
Air is separated into nitrogen and oxygen rich fractions by various cryogenic rectification processes. In accordance with one such process, incoming air, after having been compressed and cooled to a temperature suitable for its rectification, is rectified in a higher pressure column into oxygen and nitrogen rich fractions. The oxygen rich fraction is further refined in a lower pressure column connected to the higher pressure column in a heat transfer relationship. As a result of such refinement, a gaseous nitrogen tower overhead and a liquid oxygen column bottoms collect in the lower pressure column. The higher boiling components such as hydrocarbons tend to concentrate in the liquid oxygen. Argon, which has a similar volatility to oxygen, will also form part of the liquid oxygen column bottoms. Thus, the liquid oxygen produced in the lower pressure column typically is not of ultra-high purity.
In another type of cryogenic rectification process, air is separated in a single column known in the art as a nitrogen generator. In the nitrogen generator, an oxygen rich fraction is produced as column bottoms and a high-purity nitrogen rich fraction is produced as tower overhead. The oxygen rich fraction, known as crude liquid oxygen, can be used as a coolant for the head condenser at the top of the nitrogen generator in order to provide reflux for the column. After having been used to so provide reflux, the oxygen rich fraction is discharged as waste and part of it may be recompressed either at column temperature or at ambient temperature and then recycled back to the column. This type of column, although capable of producing high-purity nitrogen, is therefore not in of itself capable of producing ultra-high purity liquid oxygen.
There are plant applications that require an ultra-high purity oxygen product. For instance, in U.S. Pat. No. 4,977,746, first and second auxiliary columns are used in conjunction with a double column arrangement to produce ultra-high purity oxygen. In this patent, gas from above the liquid oxygen sump of the lower pressure column is rectified within the first auxiliary column to produce a gaseous tower overhead free of hydrocarbons. The gaseous tower overhead is then distilled in the second auxiliary column to produce ultra-pure liquid oxygen as a column bottoms. U.S. Pat. No. 5,363,656 discloses a nitrogen generator in which crude liquid oxygen is rectified in a second rectification column to separate nitrogen gas from the crude liquid. The resultant liquid oxygen is heated so as to be evaporated by a reboiler of the second rectification column and the evaporated oxygen is then introduced into a third rectification column to produce high purity oxygen gas. The high-purity oxygen gas in then introduced into a fourth rectification column so that oxygen, nitrogen, carbon monoxide and argon, are produced as tower overhead and an ultra-high purity liquid oxygen is produced as column bottoms.
A major problem in the prior an is that a large capital expenditure is required to produce the ultra-high purity liquid oxygen. For instance, in both of the above-mentioned patents, four separate distillation columns are required. As will be discussed, the present invention provides a method and apparatus for producing ultra-high purity oxygen which is particularly well adapted to be used with a nitrogen generator that is designed to efficiently produce high-purity nitrogen in addition to the ultra-high purity oxygen.